Mechanical vapor recompression evaporator systems includes a falling film evaporator used to vapourize a small percentage of brine on the tube side and to condense a pure water stream on the shell side. Currently the vapourized steam and non-vapourized brine from the tube side fall into a vertical sump which is used to separate the brine from the steam, with the brine ultimately being pumped back to the tube side of the exchanger.
The vertical sump may be the same diameter as the falling film exchanger with the steam with carried-over brine directed to a separate knockout vessel to further remove brine carry-over. The steam is then directed to a compressor (potentially going through a demister pad) where the steam is then sent to the shell side of the falling film exchanger.
Alternately, the vertical sump may be significantly larger in diameter than the falling film exchanger. In this case, the larger sump also acts as the knockout vessel on the compressor suction, and may contain a demister pad on the annulus (ring of the knockout vessel that is outside the perimeter of the falling film exchanger). Again, the steam is sent to the shell side of the falling film exchanger.
In both cases, the falling film exchanger blowdown is taken as a slipstream of the recirculating brine going from the sump to the tube side of the falling film exchanger. This brine is withdrawn from the bottom of the sump. There is no other continuous brine withdrawal of fluids from the evaporator system.
There are several concerns with the above system:                1) Carryover of brine with the steam stream is a problem. In the case with the sump being greater in diameter than the falling film evaporator, any entrained brine in the steam stream will not be removed as the steam velocity increases as it goes from the evaporator sump to the compressor suction. A proposed solution has been to put a demisting screen on the annulus for the brine droplets to contact, coalesce and fall back into sump. In practice, the brine that coalesces on the demister screens evaporates leaving dissolved solids on the demister screen surface. The build-up of solids increases the pressure drop to unacceptable levels. One proposed solution to the solids build-up was to wash the screens using the clean produced water from the evaporator. This however reduces the capacity of the evaporator. In addition, brine carryover past the demister screen results in corrosion issues in the steam ducting to the compressor and to the compressor itself.                    In the case with the sump being equal in diameter to the falling film exchanger, the volume of the sump used for steam/brine separation is the same volume in which the brine drops from the falling film evaporator must pass in order to reach the brine liquid level. There is no undisturbed volume for good separation. Secondly, the diameter of the sump is based on the size of the falling film exchanger, and not based on the separation of steam from brine. Thirdly, the total volume of the sump is significantly reduced compared to the large sump case, which reduces the steam/brine and brine/hydrocarbon separation efficiency in the sump. The use of an external knock-out drum before the compressor reduces the impact of sump design since it will be designed for liquid/vapor separation. It generally has a demisting screen on the steam outlet similar to the large sump annulus screen, but suffers the same drawbacks of requiring a clean water wash to remove solids (reduced capacity), albeit this screen will be significantly reduced in size compared to the large diameter sump. Furthermore, the brine droplets passing through the demister will still have associated corrosion problems.                        2) Hydrocarbon removal from the brine stream is problematic with both cases. Hydrocarbons in the system are normally lighter than brine and will tend to float to the top. The blowdown from the evaporator system is taken as a slipstream from the recirculating brine stream which is withdrawn from the sump at the bottom. As a result, there is a tendency for the hydrocarbons to accumulate (or at a minimum, concentrate) on top of the brine with little ability to reach the bottom of the sump where it can be blown down. The accumulation of hydrocarbons in the sump leads to foaming, additional brine carryover, and poor operation which then results in an evaporator shutdown to fully replace the brine (and accumulated hydrocarbon) with essentially hydrocarbon free brine.        3) The corollary to poor hydrocarbon removal is brine with higher hydrocarbon content. The additional hydrocarbon is not sufficient to reduce the hydrocarbon accumulation to zero, but will cause fouling and performance issues in the falling film exchanger. These are generally cleaned when the evaporator is shutdown for the hydrocarbon accumulation issues detailed in (2) above.        